This invention relates to process for the synthesis of a labeled compound used for the positron emission tomography (PET) system which is one imaging diagnostic technology and apparatus therefor.
A labeled compound used for the PET system is methyl iodide labeled with .sup.11 C which was synthesized using a synthesis apparatus as shown in FIG. 7. In this figure, the numeral 1 indicates a target gas cylinder which stores a mixed gas for the target, and the target gas cylinder 1 is connected with a target box 4 in which .sup.11 CO.sub.2 gas is produced by a transfer tube 3 through an electromagnetic valve 2. The target box 4 is connected with a collecting coil 8 by a transfer tube 7 through electromagnetic valves 5,6. A helium gas cylinder 9 is also connected with the collecting coil 8 by transfer tubes 11,7 through electromagnetic valves 10,6. The collecting coil 8 is put in a cooling vessel 12, and the outlet of the collecting coil 8 is connected with a reaction vessel 14 by a transfer tube 13. The reaction vessel 14 is further connected with a syringe 16 containing hydroiodic acid by a transfer tube 18 through an electromagnetic valve 17 as well as an exhaust tube 15.
When methyl iodide labeled with .sup.11 C is synthesized using the above synthesis apparatus, the mixed gas for the target is filled into the target box 4 from the target gas cylinder 1 by opening the valve 2. Then, proton beam 19 supplied from a cyclotron (not shown) is irradiated for a fixed time to produce .sup.11 CO.sub.2 gas through a nuclear reaction (.sup.14 N(p.d).sup.11 C). Subsequently, the target gas containing .sup.11 CO.sub.2 is delivered to the collecting coil 8 cooled in the cooling vessel 12 through the valves 5,6, and .sup.11 CO.sub.2 gas is collected. After the collection is finished, the collecting coil 8 is heated to deliver .sup.11 CO.sub.2 gas to the reaction vessel 14 by supplying helium gas from the gas cylinder 9 through the valves 10,6. In the reaction vessel 14, .sup.11 CO.sub.2 gas is reduced by bubbling it into a reducing agent solution. Then, the reducing agent solution is evaporated by heating the reaction vessel, and discharged through the exhaust tube 15. Hydroiodic acid (HI) is introduced into the reaction vessel 14 by operating the syringe 16, and methyl iodide labeled with .sup.11 C (.sup. 11 CH.sub.3 I) is synthesized. Thereafter, .sup.11 CH.sub.3 I is recovered by heating the reaction vessel 14 again to distill it. In the above process of synthesizing methyl iodide, respective times for terminating bubbling, the distillation of the reducing agent solution and the supply of .sup.11 CO.sub.2 were decided from an average time necessary for these processes which was empirically decided by adding an excess time for the security. These times were inputted into an apparatus having a time control such as a microcomputer or a sequencer as a set point, and the finish of these processes was detected.
In the above conventional process, since respective termination points of the bubbling, the distillation of the reducing agent solution and the supply of .sup.11 CO.sub.2 were decided empirically, these points were set considerably longer than the minimum time due to the variations of the bubbling time, the distillation time of the reducing agent solution and the supply time of .sup.11 CO.sub.2 caused by a delicate difference of conditions. However, the half life of .sup.11 C is about 20 minutes which is very short, and therefore, quenching of .sup.11 CO.sub.2 and .sup.11 CH.sub.3 I increased by the extension of the set points. As a result, it was difficult that both the utilization of .sup.11 CO.sub.2 and the recovery of .sup.11 CH.sub.3 I were kept always high. That is, it was impossible to satisfy both of decreasing the risk of failure in the synthesis and increasing the yield of .sup.11 CH.sub.3 I. Moreover, another problem is in a long time from the start of supplying .sup.11 CO.sub.2 to the recovery of .sup.11 CH.sub.3 I.